The present invention relates to a composition that includes an elastomer and antiozonants.
Ethylenically unsaturated elastomer compositions are particularly susceptible to ozone attack. It is known that ozone degradation will lead to cracks in articles of manufacture made from the cured composition, especially if the articles are subjected to dynamic conditions such as flexing during use.
The use of antioxidants in elastomer compositions to counter this degradation is well known. However, it is a widely held view in the elastomer formulation art that the conventional amount of antiozonant should be less than approximately 5 or 6 parts per hundred parts rubber (phr). See, for example, Morton (ed.), "Rubber Technology", p. 44 (2d ed. 1973); U.S. Pat. Nos. 5,023,287; 4,855,346; 4,559,378; 4,297,269; 4,257,468; 3,502,612; and 3,419,639; and Great Britain Published Patent Specification No. 1,172,596. Indeed, U.S. Pat. No. 5,023,287 states that "[i]t is not possible to simply incorporate higher levels of antidegradants into the rubber compound without detrimentally impacting its physical properties. Further, high levels of free antiozonant only increases the initial surface concentration above the level required for effective protection without significantly extending the time prior to failure." U.S. Pat. No. 5,023,287 also includes evidence demonstrating that "at the 8.0 phr level, the [antiozonant] compound appears to be pro-degradative and thus fails sooner than the 6.0 and 4.0 phr levels." U.S. Pat. No. 4,257,468 states that "when the amount [of the antiozonant] exceeds 5 parts by weight, the tear strength of the vulcanizate becomes low and consequently the chipping phenomenon is apt to occur."
There are a few Patents (U.S. Pat. Nos. 5,344,861; 4,794,135; 4,767,809; 3,928,344; 3,634,313 and 3,163,616 and Canadian Patent No. 748,497) that mention that up to 10 phr of antiozonant might possibly be used, but none of these Patents appear to include a specific example that includes 10 phr of antiozonant.
U.S. Pat. No. 2,877,189 describes mixing 100 g of antioxidant powder with 400 g of GR-S rubber stock and mixing 310 grams of natural rubber stock with 50 grams of antioxidant powder. Various p-phenylenediamines were used as antioxidant powders. However, these compositions were included solely for the purpose of providing a comparative example that demonstrates that the invention claimed in the'189 patent was a superior method for incorporating antioxidant into a rubber stock . There is no indication that the GR-S stock/antioxidant mixture or natural rubber stock/antioxidant mixture included a curing or vulcanizing agent or were vulcanized or cured into an article of manufacture.